A quick determination method of viscosity of lubricating oils in use by applying Fourier Transform Mid Infrared Spectroscopy method was proposed. By using this method, the standard oil samples and some lubricating oils in use were analyzed by Fourier Transform Mid-Infrared Spectroscopy. By selecting the 1 465 cm-1 as characteristic peak for the determination of viscosity, the correlations between viscosity and absorbance of the samples of standard oil and the in use oil in the characteristic peaks in the infrared spectrum were analyzed separately, thus the regression equation was given. In addition, the new oil samples of this lubricating oil were used to revise the regression equation of the standard oil. The results show that it is feasible to determine the viscosity of some lubricating oil in use by using the Fourier Transform Mid-Infrared Spectroscopy method, and the experimental results are reliable.